The B-cells of normal pancreatic islets stand are able to respond to metabolic demand by rapid insulin discharge. To ready themselves for this response, B-cells store large quantities of insulin in secretory granules, and acutely release the contents of these granules by exocytosis in a manner regulated by glucose and other secretory stimuli. In this process, B-cells synthesize proinsulin, sort it into immature secretory granules where it is converted to insulin that condenses in the mature granule core. A longstanding unexplained observation is that during granule maturation (which is continuously ongoing for a population of new granules in B- cells), a fraction of newly-synthesized insulin/proinsulin bypasses the storage pathway and is secreted in relative preference to that from mature granules containing previously synthesized hormone. Pathological inability to meet metabolic demands, such as in Type II diabetes, may result in abnormal intracellular secretory protein handling involving amplification of the preferential release process. One hypothesis has been that the cellular basis for this preferential secretion is a constitutive secretory pathway derived from the Golgi complex; however, recent studies demonstrate convincingly that < 1% of newly-synthesized proinsulin and virtually no insulin is secreted by the constitutive route. The current proposal is based on a new hypothesis that the preferential release involves the export of newly-made insulin/proinsulin via transport vesicles which bud from immature granules. It is further hypothesized that the regulation of this novel secretory pathway is linked to insulin granule maturation. Since this exocytotic pathway is derived from the same compartment where insulin condensation is taking place, we also intend to establish the relationship between intragranular insulin crystallization and the appearance of preferential newly-synthesized insulin secretion.